This invention relates to golf balls, and more particularly, to a golf ball having a core which includes polyurethane rubber.
Golf balls which are currently available fall into two general categories--balls which include a balata cover and balls which include a more durable, cut-resistant cover. Balata covers are made from natural balata, synthetic balata, or a blend of natural and synthetic balata. Natural rubber or other elastomers may also be included. Synthetic balata is trans polyisoprene and is commonly sold under the designation TP-301 available from Kuraray Isoprene Company Ltd.
Balata has been used as a cover for golf balls due to the excellent spin/playability properties and flight performance properties. However, balata is an expensive material, and processing balata golf balls is both time consuming and expensive.
Most cut-resistant covers utilize Surlyn ionomers, which are ionic copolymers available from E. I. du Pont de Nemours & Co. Surlyn ionomers are copolymers of olefin, typically ethylene, and an alpha-beta ethylenically unsaturated carboxylic acid, such as methacrylic acid. Neutralization of a number of the acid groups is effected with metal ions, such as sodium, zinc, lithium, and magnesium.
DuPont's U.S. Pat. No. 3,264,272 describes procedures for manufacturing ionic copolymers. Ionic copolymers manufactured in accordance with U.S. Pat. No. 3,264,272 may have a flexural modulus of from about 14,000 to about 100,000 psi as measured in accordance with ASTM method D-790.
DuPont's U.S. Pat. No. 4,690,981 describes ionic copolymers which include a softening comonomer. Ionic copolymers produced in accordance with U.S. Pat. No. 4,690,981 are considered "soft" ionic copolymers and have a flexural modulus of about 2800 to about 8500 psi. The disclosures of U.S. Pat. Nos. 3,264,272 and 4,690,981 are incorporated herein by reference.
Other cut-resistant materials which can be used in golf ball covers are ionic copolymers or ionomers available from Exxon under the name Iotek, which are similar to Surlyn ionomers except that acrylic acid is used rather than methacrylic acid.
Recently, ionomeric blends containing V.L.M.I. (Very Low Modulus Ionomers) have been used for golf ball covers. The addition of V.L.M.I. improves playability properties, but sacrifices coefficient of restitution as a function of initial velocity (C.O.R./Initial Velocity) and distance properties. Blends of ionomers containing V.L.M.I. are illustrated in U.S. Pat. Nos. 4,884,814 and 5,120,791.
High acid ionomers are ionomers having an acid content of 18% by weight or higher of an ethylenically unsaturated carboxylic acid. Standard grade ionomers are ionomers having an acid content of 15% by weight or lower of an ethylenically unsaturated carboxylic acid. Examples of high acid ionomers are Surlyns 8220 and 8140, which contain 20% and 19% by weight of an ethylenically unsaturated carboxylic acid, respectively.
Several patents describe using high acid ionomers to form golf ball covers. For example, U.S. Pat. No. 5,222,739 to Sumitomo Rubber Industries discloses a cover composition which contains an olefin and 20-30% of an ethylenically unsaturated carboxylic acid which has 15 to 30% of its carboxylic acid groups neutralized with monovalent or divalent metal ions. U.S. Pat. No. 5,298,572 to DuPont describes a composition formed from an ionomer or a blend of ionomers. The ionomer contains 16-25% by weight of an ethylenically unsaturated carboxylic acid which is neutralized with lithium, zinc and sodium ions.
Thermoplastic and castable polyurethane materials have been used in golf ball construction (primarily in golf ball covers) for many years, with varying levels of success.
Thermoplastic polyurethanes are produced through the reaction of bifunctional isocyanates, chain extenders and long chain polyols. To produce thermoplastic properties, it is necessary for the molecules to be linear. The hardness of the polymer can be adjusted based upon the ratio of hard/soft segments produced in the reaction. Thermoplastic polyurethanes have been evaluated as covers for golf balls, with no significant success. Thermoplastic polyurethanes generally do not have the resilience properties required for a premium sold core golf ball, and the temperature required to melt the thermoplastic polyurethanes make them unsuitable for use as covers on thread wound golf balls. Recently, there has been some success in utilizing thermoplastic polyurethanes as mantle layers in multi-layer golf ball covers.
Castable polyurethanes are made by reacting essentially equimolar amounts of diisocyanates with linear, long chain, non-crystalline polyesters or polyethers. This results in the production of a soft, high molecular weight mass with essentially no crosslinking. To solidify this material, chain extenders such as short chain diols (e.g., 1,4-butane diol) or aromatic diamines (e.g., methylene-bis-orthochloro aniline (MOCA)) are utilized. This results in creation of linear segments, which are rigid in comparison to the initial mass described above. Castable polyurethanes have been used in the production of wound golf balls for a number of years, as described in U.S. Pat. Nos. 4,123,061 and 5,334,673. However, this method of production (as descried in European Patent Application 0 578 466 A) is time consuming, and inefficient.
The vast majority of golf balls currently produced are two-piece golf balls, consisting of a solid core and a Surlyn (ionomer) cover. Generally, Surlyn covered golf balls have exceptional durability properties, but are considered hard compared to a wound ball construction, and are not preferred by the better player.
In recent years, new ionomers have been developed to result in softer feel and playability properties, but at a significant sacrifice in initial velocity and resilience properties. It is also important to note that if a very soft ionomer cover is used to lessen the compression of (soften) the golf ball, cut resistance properties also become poor.
More recently, golf balls have been introduced which utilize multi-layer covers, where a soft mantle or cover layer is used to improve the playability properties (feel--as measured by PGA compression) of the golf ball. This has been somewhat successful, but the feel (compression) of the ball can only be softened to a certain point before significant losses in resilience properties are observed. Generally, the mantle and cover layers of multi-layer golf balls are made using ionomers, thermoplastic polyester elastomers, polyether block co-polymers, and other thermoplastic materials.
The feel of a golf ball can also be improved by adjusting the composition of the solid core to produce a lower compression. Generally, a solid golf ball core is made utilizing primarily polybutadiene rubber, or a blend of polybutadiene rubber with a small amount of natural rubber, polyisoprene rubber, or both. The golf ball core is "cured" utilizing a zinc diacrylate/peroxide cure system. As the core formulation is adjusted to reduce core compression, resilience properties of the core decrease, and can decrease to a level where resilience properties are low, and unsuitable for use in a premium golf ball.